Study: Genes Influence Who Your Friends Are

You may have more in common with your friends than a shared
sense of humor or a penchant for the same bar. A new study finds that friends
often resemble each other at the level of their genomes – though certain genes
may influence people to bond with others who aren't like them.

Research has shown that birds of a feather do, in fact,
flock together most of the time: People tend to become friends with people
similar to themselves. Studies have also shown that that people's friendship
styles – the number of friends they have and how central they are in their
social network – are influenced by genetics.

These findings raised the question of whether genes play a
role in whom we choose to become friends
with, said James Fowler, a professor of political science and medical
genetics at the University of California, San Diego. Fowler and his co-author
Nicholas Christakis explored these theories in their book "Connected: The
Surprising Power of Our Social Networks and How They Shape Our Lives"
(Little Brown, 2009).

"We hypothesize that we're not just similar to our
friends socially or behaviorally," Fowler. "We may also be similar to
them biologically or genetically."

Friends and Genes

To find out if that theory holds water, Fowler and his
colleagues culled data from two large, long-running studies, the National
Longitudinal Study of Adolescent Health and the Framingham Heart Study. Both
studies collect genetic information as well as data on who knows whom through
friendship, marriage and other relationships.

The researchers tested the friendship pairs from the studies
for six genes known to influence human behavior. Most of the genes tested
influence the neurotransmitters serotonin or dopamine. (Serotonin is associated
with mood and happiness, while dopamine plays many roles in motivation,
reward-seeking behavior, learning and attention.)

People who live near each other tend to have similar genomes
simply because they're more likely to have shared an ancestry. To control for
this "population stratification," Fowler and his colleagues
controlled for ethnicity and included siblings of the subjects in the analysis.
Including data from siblings allowed the researchers to see how much of the
correlation between genes and friendship was due to population stratification
and how much was a real effect.

Of the six genes tested, two clustered among friends in
unexpected ways. The first, the DRD2 gene, codes for a dopamine receptor in the
brain, and some variants have been associated
with alcoholism in previous studies. People with similar variants of DRD2
tend to stick together, the researchers report this week in the journal
Proceedings of the National Academy of Sciences.

A second gene, the CYP2A6 gene, had the opposite effect.
People with similar variants of the CYP2A6 gene tended to befriend people with
different CYP2A6 gene variants. Little is known about this gene, the
researchers wrote. It's responsible for a protein that metabolizes several
compounds, including nicotine. One earlier study associated the gene with personalities
that are open to new ideas.

A sea of genes

While the CYP2A6 gene effect is somewhat mysterious, it's not
hard to imagine that a person
with alcoholic tendencies might be drawn to people and places that
non-drinkers would avoid, Fowler said. Researchers can't yet say if people's
genes influence their friendships mostly by pushing people into environments
with like-minded (and like-genomed) individuals, or if people are choosing pals
based on geneticallyinfluenced
personality traits. The finding that opposite CYP2A6 variants attract suggests,
however, that some individual choice is involved, Fowler said.

"With the negatively correlated gene, we can rule out
the possibility that you and I have been drawn to the same environment because
we have the same genotype, because we don't have the same genotype," Fowler
said.

The findings are a "first step," Fowler said. He
and his colleagues hope to repeat the study with whole-genome scans to test all
25,000 genes humans are estimated to have instead of just the six the
researchers initially tested.

If the results hold (or if more influential genes are
discovered), they could add another wrinkle to the mystery of
gene-environment interaction. If genes influence a person's social
environment, that social environment could, in turn, influence the person's
behavior: Imagine an alcoholic who befriends other alcoholics. Hanging out in
an alcoholic crowd could encourage the person to drink more.

But people may also choose friends under the subtle
influence of evolution, Fowler said. Previous research has found that people tend to
choose spouses with different immune-system genetics than their own,
perhaps to increase their protection against contagious diseases (if you can't
fight off a bug, you want to be sure your partner won't get ill and transmit
that same bug to you). Perhaps the CYP2A6 gene, while not linked to the immune
system, plays a similar protective role.

"We live in a sea of the genes of others," Fowler
said. "We are not just influenced by our own genes… We think that we are
going to find more and more biological processes underlying the social networks
that we live in."

Stephanie Pappas

Stephanie Pappas is a contributing writer for Live Science. She covers the world of human and animal behavior, as well as paleontology and other science topics. Stephanie has a Bachelor of Arts in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz. She has ducked under a glacier in Switzerland and poked hot lava with a stick in Hawaii. Stephanie hails from East Tennessee, the global center for salamander diversity. Follow Stephanie on Google+.